1. Field of Invention
The present invention relates to power conversion, and more particularly, to dynamic selection of oscillation signal frequency for a power converter.
2. Description of Related Art
Power converters are essential for many modern electronic devices. Among other capabilities, power converters can adjust voltage level downward (buck converter) or adjust voltage level upward (boost converter). Power converters may also convert from alternating current (AC) power to direct current (DC) power, or vice versa. Power converters are typically implemented using one or more switching devices, such as transistors, which are turned on and off to deliver power to the output of the converter. Control circuitry is provided to regulate the turning on and off of the switching devices, and thus, these converters are known as “switching regulators” or “switching converters.” Such a power converter may be incorporated into or used to implement a power supply—i.e., a switching mode power supply (SMPS).
In various applications, it is desirable for power converters to be configured to address particular concerns. For example, in portable, battery-operated wireless communication devices (such as cellular telephones, personal digital assistants (PDAs), and laptop computers with wireless capability), it is important for a power converter to be configured to optimize battery life and not generate noise which might interfere with the transmission spectrum. In such communication devices, radio frequency power-amplifiers (RFPAs) are one component which consume a substantial amount of power. Indeed, RFPAs can have such power-intensive demands that they may dominate power consumption over all other components in the systems.
In communication devices implementing or using second generation (2G) and third generation (3G) protocols or standards (e.g., GSM 900, GSM 1900. DCS 1820, UMTS FDD W-CDMA, UMTS Satellite, WiFi 802.11, etc.), high efficiency dynamic voltage biasing (DVB), or Envelop Tracking (ET) power supplies are required to provide or supply power to RFPAs in order to achieve high power efficiency. A DVB power supply is a high-efficiency switched regulator (such as a buck or buck-boost DC-to-DC converter) that adjusts the DC supply level for power to the RFPA according to a desired RF power level. A DVB power supply achieves high power efficiency and increases the autonomy of the battery.
Typically, DC-to-DC converters operating at 1 or 2 MHz are used for such DVB or ET power supplies. However, such converters can be problematic due to their output voltage ripple or noise, which interferes with the output carrier spectrum and creates sideband spurs that degrades Signal/Noise ratio for the adjacent channels. Actually, the RFPA is a nonlinear device (especially at high output power level) which causes it to act as a frequency mixer—it mixes the RF input spectrum with the supply voltage spectrum. This results in up converting the DC-DC noise from few MHz to spurs that are placed next to the RF carrier. These spurs can not be filtered because they are too close to the carrier signal.
One way to reduce such noise is to use filtering and electromagnetic interference (EMI) shielding between the SMPS and the RF circuits. But noise filtering and EMI shielding at low switching frequency can require relatively large components and hardware to implement, thus occupying valuable space in the communication device and potentially affecting the size of the device. Furthermore, it is difficult to model the interference paths between the DC-to-DC converter and the transmission spectrum, and to determine with any reasonable degree of accuracy the location and magnitude of the coupling between the power supply and the radio-frequency (RF) components (e.g., RFPA, phase locked loop (PLL), mixer, low-noise amplifier (LNA), etc.). Indeed, analysis for noise filtering is still empirical and requires a very good understanding of the RF circuit architecture, the system noise immunity, and the RF frequency standards.